1. Field of the Invention.
The invention relates generally to High Electron Mobility Transistors (“HEMTs”). More particularly, the invention relates to epitaxial nucleation and buffer sequence for via-compatible InAs/AlGaSb HEMTs.
2. Description of Related Art
When electrons from ionized donors are placed in a material with higher conduction band energy than the channel, and close to the channel, they create an electron gas with high electron mobility. Transistors that implement this concept are known in the art as High Electron Mobility Transistors (“HEMT”).
A HEMT is a field effect transistor with a junction between two materials of different band gaps, the barrier material and the channel material. The barrier material contains donor electrons with higher conduction band energy than the channel material. This improves the electron mobility by separating the donor ions from the conducting channel. The channel material makes up the conducting layer and is selected based on the transport properties of the electrons, with the band gap also being a consideration to support high fields and high voltages. Conventionally, a HEMT system is made from Galium Arsenide (GaAs) with Aluminum Galium Arsenide (AlGaAs). The donor-containing wide bandgap material is AlGaAs and the conducting channel is GaAs. The effect of the junction between these two materials is to create a very thin layer where the Fermi energy is above the conduction band, giving the channel very low resistance and high electron mobility.
HEMTs can be used for Monolithic Microwave Integrated Circuits (MMICs). In most cases, MMICs require a semi-insulating substrate to allow the use of microstrip transmission lines and high-Q passive elements such as integrated inductors. MMICs also require a semi-insulating substrate to reduce the substrate loss at high frequencies.
Ordinarily, the substrate material and the HEMT materials have the same lattice constant. If the two materials have a different lattice constant, a buffer layer can be placed between them to form a metamorphic HEMT (“mHEMT”). Since InAs-channel HEMT circuits have no lattice-matched substrate that is semi-insulating, all practical InAs-based HEMTs have been grown via metamorphic growth on a semi-insulating substrate with a different lattice constant, usually GaAs.
Prior art metamorphic buffers have had thick layers (>1 micrometer) of Aluminum Antimonide (AlSb) or Gallium Antimonide (GaSb). Since a GaSb buffer is too conductive for RF circuits, prior art InAs/AlGaSb HEMTs have typically been grown using an AlSb metamorphic buffer layer. The use of pure AlSb for the majority of the metamorphic buffer presents a problem in integrated circuit fabrication because pure AlSb is very unstable and is prone to oxidation and subsequent cracking of the epitaxial material after AlSb has been exposed to chemicals, such as acid, base, cleaning solvents, water, and even air after several hours of exposure. FIG. 1 is a cross-sectional photo obtained using a secondary electron microscope (SEM) and illustrates the oxidation of the AlSb buffer.
There are several integral steps in MMIC production that expose the AlSb metamorphic buffer to chemicals, including the etching of vias connecting the front and back sides of the wafer and the cleaving of the wafer into individual chips which expose the AlSb at the sidewalls. With an increasing demand for improved MMICs, there remains a continuing need in the art for an epitaxial nucleation and buffer layer sequence that is stable when exposed to chemicals.